To understand firewalls, you should understand how the basic communication with other computers/servers on the internet work. When you connect to the Internet and use various software like Internet Explorer, Netscape, Outlook and other email applications, lots of information is being transmitted to and from your PC over a single connection. How does this information coming into your pc know to go to your email software for email and to your web browser for web site display? How is an outgoing message distinguished from an incoming message? Basically, it is done through standard ports. A port is a doorway into and out of your PC and TCPIP can direct the information to a specified port. Your TCPIP address is the location of your computer on the Internet and the port number is the specific door to use at this address.
Ports are numbered and there are over 6000 available ports to address. These port numbers are only effective when your PC and the one you are connecting to are both attempting to use the same port number. If they are not they can not communicate with each other. So, there had to be some sort of advance agreement on what applications will use what port number. For example, there was an agreement when the World Wide Web was being developed that all browsers will use port 80. That allows web sites to set their web servers to communicate it's hosted websites on port 80 understanding that Microsoft, Netscape and all other brands of browsers would program their software to accept and transmit traffic on the same port 80. So, anything that comes in to your computer on this port is directed to your web browser and allows the website to be displayed on your PC.
The developer's of email agreed to set port 110 as the standard port for incoming pop3 email and outgoing smtp email uses port 25. By agreeing on these port numbers, all email software developers program their software to use these ports and allows your software to communicate properly with incoming and outgoing mail servers to make your email work. In a nutshell, the ports open the doors (establish the paths) of communication between two computers. TCPIP uses the open door to send it's packets of information between to two pieces of software that wish to talk with each other.
By default, when you connect to the Internet all of these 6000 plus doors are open and allow free access to your PC. If you think about this you can see that this is a pretty scary thought since not everyone that connects to the Internet has good intentions. This allows a dishonest computer programmer the ability to program a server type application on his pc to communicate on any of the 6000 plus ports and plant a hidden application on your pc to use the same port number and then send whatever instructions he wants to your pc for execution. This is the basic premise of what you hear are "Trojans".
Now, you might ask, "I'm one of millions of PC's on the Internet. Why would someone want to attack me? I have nothing of value to offer." Well, remember, with the open door, they can come in and plant a little webserver on your PC and you will never know it. By use of the hidden file attribute they can copy images to your computer and through the use of re-directs your PC can become a popular porn site on the Internet and you would never realize it is happening.
Enough of the scare tactics, let's do something simple about this and slam the doors shut. First, let's see how vulnerable you are right now. Go to the
Shields Up site and check your present security. When you get there, click the "Proceed" button and that will take you to the greetings page. After reviewing the information look at the blue box on the page and you should see a series of silver buttons labled File Sharing, Common Ports, and all Service Ports among a few others. Be sure and check each of those three buttons and review the information that is returned. This will give you a clear indication of how secure your PC is on the Internet.
A firewall is a piece of software that runs on your PC and allows you to close any open port that you wish to close. Normally by default it closes all but the standard ports used to communicate to web browsers and email clients. Firewalls can be configured many different ways. You can allow certain ports to let traffic in but not let it out on certain ports. You can open it both ways or block it both ways.
Firewalls can be obtained through several sources. Many of the major Anti-Virus software companies like Norton, McAfee and Trend are building firewalls into their software and marketing them as "Internet Security Suites". These are quite good and offer a total package that covers most of the other topics I will mention such as Virus, Spyware and Spam control. If you do not have one of these suites and need a firewall for your PC you can get a very good free one at
Zonelabs.com.